Self‐Doped Small‐Molecule Electron Transport Layers: Bay‐Modified Perylene Diimide Derivatives for Thickness‐Insensitive Organic Solar Cells

To attain commercially viable organic solar cells (OSCs), the development of electron transport layers (ETLs) with robust thickness insensitivity is of utmost significance. To achieve this, two perylene diimide (PDI) derivative ETLs, PDI-SFXA and PDI-NHSF, are synthesized. First, compared to PDI-NHSF, theoretical calculation manifests that thiophene-amide groups in PDI-SFXA have stronger electron-donating properties, promoting charge transfer and enhancing n-type self-doping. Specifically, the intensity of the electron paramagnetic resonance signal of PDI-SFXA is over six times higher than that of PDI-NHSF. Second, PDI-SFXA has a longer conjugate length than PDI-NHSF, which gives better conductivity and electron mobility. These factors confer superior thickness insensitivity to PDI-SFXA. Notably, even with a thickness of 40 nm, PDI-SFXA can maintain more than 90% of its peak power conversion efficiency (PCE), while the performance of PDI-NHSF deteriorates dramatically when the thickness is 30 nm. In devices with active layers based on PM6:Y6 and PM6:L8-BO, the efficiencies of PDI-SFXA are superior to those of PDI-NHSF, and it retains more than 90% of its initial PCE after 180 h. This work demonstrates that the combined effect of bay position modification and n-type self-doping remarkably enhances both the thickness insensitivity and long-term stability of OSCs.